Macbook Air A2337 and HDR output problems

I have a brand new MacBook Air (Retina, 13-inch, 2020) A2337.

I'm trying to get it to output in HDR to a couple of different external monitors/TVs without success.

When using a StarTech USB-C Display Adapter (CDP2HD4K60H) to connect to my monitors by HDMI I get no option to enable HDR in Display Settings at all. Both my displays support HDR, have ultrawide colour enabled, and I have tried three different quality Ultra High Speed HDMI cables.

I also have a different USB-C to DisplayPort (not sure of the brand) that works just fine and lets me enable HDR in the settings.

This, despite being the exact opposite of what Apple documents here! (they say that DisplayPort is not supported, but HDMI is) -

https://support.apple.com/en-ca/HT210980